How to convert image textures to different formats?
In the UV/image Editor select the texture or image you want to convert and press F3 (or Image->Save As),



Then select which format you want to save as.

(If you don't see these options on the left side of the screen press T.)



NOTE: Gif files are not supported in blender..

see http://wiki.blender.org/index.php/Doc:2.6/Manual/Data_System/Files/Formats